I'm designing one of my first layouts and I've read a lot of articles and StackExchange questions about guidelines and recommendations.
It's a university project realized by another student that I had to correct for some major mistakes. I've corrected those and now I'm trying to improve the layout in general. This extra work is not really required, but anyway I'm trying to make things how they should be done to learn more.
- C18 and C19 are 100nF, required by the DDS for input AC coupling.
- R25 and R26 are 50 Ohm terminator resistances placed there I suppose because they where present in an example of the oscillator datasheet.
- C_TA13 is a 10uF decoupling capacitor.
- R25, R26, C18 and C19 are 0805 standard package.
- Are R25 and R26 terminator resistances really necessary? (see Update 1)
- Is 10uF for C_TA13 too high value for its purpose?
- Assuming the schematic is right, I feel like I've done a terrible job with the layout. I've tried to connect all supply and ground pins together before going to the ground and power planes with vias only at one point. Do you see any problems with this layout?
- Should I connect the oscillator ground with the DDS ground on the top layer and then connect to the ground plane only at the thermal pad of the DDS?
- The DDS has an input impedance of 1500 Ohm and an internal bias of 1.15V (hence the AC coupling). Should I terminate the line before the capacitors even if there is an impedance mismatch at the receiver?
- This Application Note pictures a really worring scenario caused by paralleling decoupling capacitors, especially if very far away in value (decades like in the suggestion from The Photon and in other common answers). Should I be really concerned and carefully simulate the real capacitors in parallel, or is that A.N. exaggerating?
- I'm insisting with the "single via to the ground and power planes, just after decoupling" concept, because I'm trying to avoid the center-fed patch antenna that Olin Lathrop always points out in his answers. This way I try to keep fast switching currents away from the ground plane. I understand that using multiple vias for the oscillator pins and terminator resistors would defeat the purpose. Is it correct?